Fiber optic systems utilize light transmitted through an optic fiber to communicate information. Fiber optic systems are used in many applications, from the relatively simple, such as a fiber optic telecommunications system, to the complex, such as an optical computer. Fiber optic systems have many advantages over electronic systems, including, greater information carrying capacity and smaller size, as well as being resistant to electromagnetic interference and relatively inexpensive to fabricate and install.
To illustrate the general operation of a fiber optic system, the operation of a fiber optic communication system is described. Fiber optic communication systems operate by modulating a received electronic signal, such as a phone call, into an optical signal. The optical signal is transmitted through the optic fiber to its destination. The optical signal is then demodulated, and an electronic signal having the same characteristics as the received signal is output.
Fiber optic communication systems generally comprise a transmitter, an optic fiber, and a receiver. The transmitter receives an electronic signal and an input light beam. The input light beam is a beam of light produced by a laser with constant optical intensity. The optical intensity of the input light beam is modulated, or varied, in direct response to the electronic signal to produce an optical signal. The optical signal is then transmitted through the optic fiber to the receiver. The receiver includes photodetectors that convert light into electrical current in direct proportion to the optical intensity of the light striking the photodetectors. Accordingly, the variation in the optical intensity of the optical signal produces a corresponding variation in the electrical signal produced by the photodetectors, thereby demodulating the optical signal into an electronic signal.
A technical disadvantage is that the variation in optical intensity of an optical signal due to the input electrical signal is relatively small in conventional optical modulators. The greater the variation in the optical intensity (modulation depth), the greater the variation in the electrical signal produced by the optical receiver. Accordingly, the greater the modulation depth relative to the system noise, the fewer the transmission errors.
Another technical disadvantage of conventional fiber optic systems is that the optical signal must often be conditioned to correct for attenuation and dispersion of the optical signal as the optical signal travels through the optic fiber. Conditioning is often performed by optical repeaters that are placed in-line with the optic fiber to receive and repeat the optical signal. The distance between optical repeaters is generally a function of the strength of the modulated optical signal. The greater the modulation depth of the optical signal, the greater the spacing of optical repeaters, and the less the cost associated with the fiber optic system.